Here's a shot of a swale I cut on my place last year. This was taken in early winter and the grass has just started to grow.
There is a natural slope coming from the right (just out of frame), so I cut the swale at an angle to get as much water as I could to the left hand side before it drains down into the holding area at the bottom. (It's relieved by a trench cut with the backhoe and drains out into the trees in the background.)
I cut this with the box scraper. Put an angle on the scraper and worked with that 'til I had the grade I wanted in the bottom of the swale, then levelled it out and kept going deeper. I cut the angles on the edges by running my left wheels on the top of the bank and running my right hand side in the bottom of the cut. At this angle, I can ride the lawnmower down the sides to keep things trimmed up. That's a good thing. I'm lazy. I don't like pushing hand mowers.
Slim